System and method for synchronizing networked components

ABSTRACT

A method for synchronizing a plurality of components that are networked via a plurality of high speed switches, the method includes frequency-locking to a master clock component clocks of the plurality of components, and synchronizing to a master counter, driven by the master clock, component counters of the plurality of components, so that the frequency-locked component clocks drive the component counters in synchrony with the master counter.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.15/260,664 filed on Sep. 9, 2016, which is a divisional of U.S. patentapplication Ser. No. 14/051,137 filed Oct. 10, 2013, both of which arehereby incorporated by reference in their entirety.

BACKGROUND Technical Field

Embodiments of the invention relate generally to time synchronization ofnetworked components. Particular embodiments relate to timesynchronization of CT scanner subsystems and computer equipment.

Discussion of Art

CT scanners are used in medicine both for diagnosis and for guidance ofinterventions. In a CT scanner, an X-ray source and a detector (such asan array of scintillation crystals with associated photomultipliers orphotodiodes) are positioned opposite each other on a gantry, whichrotates around a target such as a patient. The X-ray source is activatedto emit X-rays, some of which pass through the target and others arereflected or absorbed by the target. X-rays that pass through the targetto the detector generate signals.

The signals at the detector vary temporally due to stochasticfluctuations in absorption and reflection of the X-rays, and also varyspatially as the source and detector rotate around the stationarytarget, which thereby presents different absorption and reflectioncharacteristics at each angular position of the gantry. The variationsof the detector signals, during one or more rotations of the gantry, canbe mathematically correlated or “computed” in a central processing unit,which generates correlation data suitable for producing across-sectional image representation of different types of tissue orother material within the target.

Computer processors are known for implementing software, e.g., thealgorithms necessary for operation of a computed tomography (CT) scannercomponents to emit and detect X-rays and to correlate detections withgantry positions and with source position relative to the target.Computer processors implement software instructions by switchingelectronic logic gates to manipulate binary HIGH or LOW signals(“bits”). Computer processors can be implemented in RISC, ASIC, FPGA, orother architectures.

Accuracy of correlation, and, consequently, image quality, depend to agreat extent upon accurate logging of the X-ray source and detectorpositions relative to the target. Typically, these positions arecalculated based on elapsed time from a zero-time reference position,and based on measurements of gantry angular velocity derived from, e.g.,a time integral of gantry motor current, voltage, or shaft speed.Alternatively, positions may be directly measured from a bar code or thelike marked on a surface of the gantry. In any case, each detectorsignal and each position reading typically is tagged with the local timeat the detector or position sensor, and for enhanced quality, the X-raysource power level also may be continuously or periodically measured andtagged with the local time at the source. Thus, timekeeping is acritical aspect of CT scanner design.

In complex systems like CT scanners, multiple processors are networkedtogether. For example, a CT scanner may include an X-ray sourceprocessor, an X-ray detector processor, a position sensor processor, anda gantry control board processor, all of these being configured forcommunication with each other via a network. Typically, networkedprocessors communicate with each other by sending data packets. Forcoordinated action, networked processors attempt to maintain synchronyso that each received data packet is implemented or passed on in propersequence. Each processor has its own clock (comprising a time counterdriven by an oscillator, e.g., a bistable), and synchrony can bemaintained by periodically transmitting and receiving a time signal,from a designated “master” clock to the “slave” clocks of the otherprocessors. The component clocks periodically adjust their time countersin response to the time signals received from the master clock. Betweentime signals, the component clocks increment their time counters basedon the pulses produced by their respective oscillators.

The master clock/time signal paradigm is often employed because it isdata-efficient and has low network overhead. Typical periodicities formaster clock time signals range from about 1 MHz to about 200 MHz andeach processor switches its logic gates anywhere from about 1000 timesto about 5 times between receipt of each time signal from the masterclock. As a result, a large quantity of process data can be generatedand transmitted between time signals, using the same signal path usedfor the time signals (“path sharing”). By contrast, continuoussynchronous network time protocols (continuous time signaltransmissions) are not favored because such protocols require adedicated physical layer that cannot also be used for transmittingprocess data.

A potential issue with the time signal paradigm is that, during thenumerous switching operations between time signals while the componentclocks increment their respective time counters, asynchrony or clock“drift” can emerge between the master clock and each component clock andalso among the component clocks. The clock drift phenomenon arisesbecause each component clock oscillator has its own characteristicfrequency that infinitesimally varies from the frequencies of the otherclock oscillators. Over an extended sequence of switching operations,the different frequencies of the different oscillators can result insignificant divergence of the component clock time counters.

Another potential issue with the time signal paradigm is that a uniquenetwork transit time or signal delay exists between the master clock andeach component clock. Accordingly, each component clock receives thetime signal at a different time relative to the master clock. This timedifference can change according to environmental factors that can varythe transmission delay. Some possibly relevant environmental factorsinclude, for example, thermal strain, changes in impedance,electromagnetic field interactions. The ordinary expected result of thisvariation in transit time is that each component clock time “jitters”around the master clock time.

For example, image quality of a CT scanner can be limited by “clockdrift” or “jitter” between the processors associated with the detector,the source, the position sensor, and the gantry control board. Inparticular, variance of detector local time away from source or positionsensor local time, or from gantry control board local time, can resultin erroneous position readings, which limit the achievable quality ofthe correlation data. Depending on the error threshold designed into theparticular CT scanner, small clock drifts can degrade image qualitywithout establishing an error condition.

IEEE 1588 improves on the time signal paradigm by implementing a BestMaster Clock (BMC) algorithm that is run by all of the processors, inorder to select which processor will run the master clock. One featureof the BMC algorithm is that the processors cooperatively select amaster clock in a manner that is not necessarily determinative from auser standpoint—in other words, the BMC algorithm is not intended to“force” selection of any particular processor as the master clock.

IEEE 1588 addresses an issue of component clock drift due to componentclock oscillator frequency differing from master clock oscillatorfrequency by instituting a reciprocal measure of time signal delay. Thereciprocal measure of time signal delay is used by each component clockfor adjusting its local time from the time of receiving the master clockpulse, to more closely approximate the simultaneous local time at themaster clock. Determining the reciprocal measure of time signal delay,for each component clock, requires repeated reciprocal communication atscheduled intervals between the master clock and the component clocks.

BRIEF DESCRIPTION

An aspect of the invention provides a method for synchronizing aplurality of components that are networked via a plurality of high speedswitches. The method comprises frequency-locking to a master clock thecomponent clocks of the plurality of components, and synchronizing to amaster counter, driven by the master clock, a plurality of componentcounters driven by the plurality of component clocks, so that thefrequency-locked component clocks drive the component counters insynchrony with the master counter.

In another embodiment, a method is provided for frequency-locking acomponent clock to a master clock. The method comprises defining a clockfrequency and a master counter by periodically providing a master clockpulse from the master clock to a clock encoder; in the clock encoder,modulating the master clock pulse onto a serial data stream that has abit transition rate greater than the clock frequency; and transmittingthe clock stream from the clock encoder to a clock decoder portion ofthe component clock, which recovers clock pulses from the clock stream.

Embodiments of the invention provide a synchronous network time system,which comprises a master clock configured to generate a sequence ofmaster clock pulses at a clock frequency; a master processor configuredto receive the master clock pulses, maintain a master counter of themaster clock pulses, and generate a serial data stream includinginstructions for initializing a component counter; a clock encoderconfigured to receive the serial data stream and the master clockpulses, and generate a clock stream by modulating the master clockpulses onto the serial data stream; a clock decoder configured toreceive the clock stream, and extract from the clock stream the serialdata stream and a sequence of component clock pulses; and a componentclock configured to initialize and maintain a component counter based onthe instructions from the master processor and the component clockpulses.

Moreover, a technical effect of the invention is to provide “zerooverhead” modes of synchronizing networked components. As used herein,the term “zero overhead” refers to methods and systems for synchronizingnetworked components that do not require repeated discrete time signalsor a dedicated channel for continuous time signaling (once originallysynchronized).

DRAWINGS

The present invention will be better understood from reading thefollowing description of non-limiting embodiments, with reference to theattached drawings, wherein below:

FIGS. 1 and 2 are front and side elevation views of a conventional CTscanner.

FIG. 3 shows in schematic view a mode of clock frequency communication,according to one aspect of the present invention.

FIG. 4 shows in schematic view a process for clock synchronization,according to one aspect of the present invention.

FIG. 5 shows in schematic view details of steps for determining clockerrors, according to the process of FIG. 4.

DETAILED DESCRIPTION

Reference will be made below in detail to exemplary embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference characters usedthroughout the drawings refer to the same or like parts, withoutduplicative description.

Aspects of the invention relate to driftless synchronization of multipleclocks implemented in digital circuitry. Although exemplary embodimentsof the present invention are described with respect to a CT scanner thatincludes a gantry control board and plural gantry components mounted ona gantry within a stationary frame, embodiments of the invention alsoare applicable for use with networked time systems, in general.Networked time systems may be found, for example, in engine controllersfor automotive, off-highway, marine, or rail vehicles, as well as inpower generation and distribution systems.

Referring to FIGS. 1 and 2, CT scanners 10 typically include a source 12and a detector 14, which are supported on a rotating gantry 16. Thegantry is supported in a frame 18, which is pivotally mounted onsupports 20. The gantry 16 is rotatable within the frame 18 by pluralmotor-driven wheels 22, which may be mounted on the gantry or on theframe. The source 12, the detector 14, and the motor-driven wheels 22(collectively, “gantry components”) each are controlled by processorsthat implement instructions transmitted from a gantry control board(“master endpoint”) 24, which in turn is configured to implementinstructions transmitted from a system controller 26.

CT scanners 10 further include a table 28, which is configured anddisposed to be longitudinally movable generally parallel to the axis ofrotation of the gantry 16. The table 28 is operated separately from thegantry components, typically under control of the system controller 26.

In use, a patient P is positioned on the table 28 within the gantry 16,and the gantry control board 24 operates the wheels 22 to rotate thegantry around the patient. As the gantry 16 rotates, the gantry controlboard 24 also operates the source 12 and the detector 14. The gantrycontrol board 24 correlates time-stamped detector data with time-stampedgantry position data, thereby obtaining scan data, which the gantrycontrol board then passes to the system controller 26. Simultaneously,the system controller 26 operates the table 28 to either continuously orstepwise move the patient P along the gantry rotational axis, andreceives time-stamped table position data. The system controller 26correlates the table position data with the scan data from the gantrycontrol board 24 to generate either a set of one or more “slice” scansclosely spaced along the patient, or a continuous “spiral” or “helical”scan around and along the patient. The system controller 26 thentransmits the scan or scans to an off-board processing unit (not shown),which produces viewable scan images as well as a 3-D model of thepatient.

As discussed above, a longstanding issue in CT scanning technology isthat clock drift, among the gantry components and the gantry controlboard 24, can potentially degrade the accuracy of the scan data.Additionally, clock drift between the gantry control board 24 and thesystem controller 26 can potentially degrade the accuracy of the scanimages and of the 3-D model.

Referring now to FIG. 3, it has been typical that the gantry controlboard 24 communicates with at least one of the gantry components 14, 16,22 or the system controller 26 via electrical contact between a pair ofslip rings 29 (shown schematically in FIG. 3), which are disposed on thegantry 16 and on the frame 18. In any case, according to aspects of thepresent invention, a synchronous network time system 30 is implementedto frequency-lock all the components to a master clock frequency of thegantry control board, and thereby eliminate clock drift among the gantrycontrol board and the gantry components.

In order to support clock frequency communication or“frequency-locking,” as shown by FIG. 3, the gantry control board (moregenerally, “master endpoint”) 24 includes a master processor 31, amaster switch 32, a reference oscillator or “master clock” 34, a clockencoder 36, and a master counter 37 that is maintained in the masterprocessor 31. The master processor 31 is a processor, e.g., an ASIC,FPGA, or conventional processor (typically operating at a relativelyhigh instruction throughput, e.g., 8 gigaflops or higher) that generatesa relatively high speed serial data stream (the “master stream” 33)encoding instructions for operation of subordinate components. Themaster switch 32 is, by way of example without limitation, a switchcompliant with the Serial Rapid IO standard. In certain embodiments, themaster switch 32 is capable of handling a data throughput of, e.g., atleast about 1 Gb/sec. (As used herein, the terms “substantially,”“generally,” and “about” indicate conditions within reasonablyachievable manufacturing and assembly tolerances, relative to idealdesired conditions suitable for achieving the functional purpose of acomponent or assembly.) The master clock 34 is connected incommunication with the master switch 32 and the master processor 31 toprovide both devices a “master clock pulse” 35 at a pre-determined clockfrequency, e.g., in some embodiments between about 60 MHz and about 270MHz, or in other embodiments between about 100 MHz and about 180 MHZ, orin certain embodiments about 125 MHz. Other data rates and frequenciesmay be used. The clock encoder 36, typically implemented within themaster switch 32, but also implementable in a separate device, e.g., anFPGA, an ASIC, a generic processor appropriately configured by software,or the like, encodes the master clock pulse 35 onto the master stream33, thereby producing a clock stream 38 that is outgoing from the masterendpoint 24 to subordinate components 39. The subordinate components 39may include “endpoint” components (e.g., the source 12 or detector 14)as well as “intermediate” components (e.g., a second switch similar tothe master switch 32). Each of the subordinate components 39 includes aprocessor 40, which may be configured to accomplish a particularfunction (e.g., controlling the source 12 or the detector 14) or simplyto route data (e.g., a second switch as mentioned above).

Therefore, while the CT scanner 10 is operating, the master processor 31continually generates the master stream 33, which includes a pattern ofbit transitions at relatively high data rate (e.g., from about 2.5 G/bsup to about 5 Gb/s, or typically 10× to 20× the reference clock rate).The master stream 33 includes instructions from the master processor 31to each of the various component processors 40. The clock encoder 36,which is implemented in the master switch 32, encodes each master clockpulse 35 into the master stream 33 in order to produce the clock/datastream 38.

Meanwhile, each of the components 39 includes a processor 40, anoscillator 41, a clock decoder 42, and a cleanup phase-locked-loop(“PLL”) 46. Each of the components receives from the master endpoint 24,at its respective clock decoder 42, the clock stream 38, which encodesthe master clock pulse 35. The clock decoders 42 receive and demodulatethe inbound clock stream 38 in order to reproduce the master stream 33and in order to obtain demodulated or decoded clock pulses 45. Eachclock decoder 42 passes the demodulated master stream 33 directly to thecomponent processor 40. Because the demodulated clock pulses 45 mayinclude some artifacts of modulation/demodulation, e.g., partial phaseshifts, spurious signal level changes, or other noise attributable toencoding, transmission, and decoding of the clock pulse (collectively,“jitter”), the clock decoder 42 passes the demodulated clock pulses 45to the component processor 40 via the PLL 46, which removes the jitterto produce a component clock pulse 47. The PLL 46 then passes thecomponent clock pulse 47 to the component processor 40, and back intothe clock decoder 42. In view of the above, the clock decoder 42, andthe PLL 46, together form a component clock 48 that is driven by theclock stream 38. In other words, once the component has started up, theclock stream 38 replaces the component oscillator 41.

In case one of the components 39 is an “intermediate” component (i.e., aswitch), the processor 40 is configured to simply encode the componentclock pulses 47 back onto the extracted master stream 33, therebyproducing an outbound copy of the clock stream 38. On the other hand, incase another of the components 39 is an “endpoint” component, then thecomponent processor 40 implements a component counter 43 that is driven(incremented) by the component clock pulses 47 from the component clock48 in order to indicate local time at the component.

Under the conventional “time signal” paradigm, every component would beoperated such that its respective oscillator 41 would increment acomponent counter, which would be periodically adjusted based on a timesignal sent from a master clock. By contrast, according to aspects ofthe present invention, the master clock 34 and the component clocks 48together form a global time keeping system that transparentlysynchronizes the master and component counters 37, 43 without requiringrepeated broadcasts of discrete time signals or a discrete channel forcontinuous time signal communication. Thus, embodiments of the inventionprovide a “zero overhead” paradigm for synchronization of networkedcomponent clocks.

FIG. 4 shows in schematic view a sync process 50 that the masterprocessor 31 and the endpoint component processors 40 implement forinitializing the component counters 43 in synchrony with the mastercounter 37. This sync process 50 is performed at each system startup,and on detection of a newly-connected endpoint component. In the syncprocess 50, the master endpoint 24 inserts 52 into the master stream 33an instruction 53 to “enable endpoint sync.” In response to theinstruction 53, the master processor 31 sends 54 an instruction 55 to“squelchallnodes.” This causes all receiving components to suspendnormal operations until the time synchronization process 50 has finished(or, optionally, until expiry of internal timers).

The master processor 31 waits 56 for a pre-determined time, then sends58 a packet 59 “unsyncresponder.” Each endpoint component processor 40receives 60 “unsyncresponder,” and accordingly, zeroes or clears orotherwise desynchronizes its associated component counter 43. Then, eachendpoint component processor 40 returns 62 a packet 63“returnunsyncresponder.” After receiving “unsyncresponder” back from allthe endpoint component processor 40, the master processor 31 sends 64 aping packet 65 “setresponderclose,” which encodes a time of sending fromthe clock encoder. Each endpoint component processor 40 receives“setresponderclose” and sets 66 its component counter 43 to the encodedtime of sending.

Referring to FIG. 5, each endpoint component processor 40 now has itscomponent counter 43 set to a value that lags the master counter 37 byan unknown time error 68. Thus, the master processor 31 begins todetermine 69 the plurality of time errors 68, each corresponding to oneof the clock decoders 42, so that the component counters 43 can be setto better approximate the master counter 37. Repeatedly, the masterprocessor 31 sends 70 to each endpoint component processor 40 a“pingnothing” packet 71, and stores the time of sending “pingnothing” as“send_time” 73. Each endpoint component processor 40 returns 74“returnpingnothing” packet 75, which encodes a time of sending from theendpoint component processor 40 as obtained from the associatedcomponent counter 43. Typically, and on average, the time of sendingencoded in “returnpingnothing” 75 will match the receipt time at whichthe clock decoder received “pingnothing” 71. The master processor 31stores 76 its time of receiving “returnpingnothing” as “received_time”77, and the clock decoder time of sending as “responders_time” 79. Inview of the above, “receipt time” as used herein refers to the timesthat the clock decoder receives “pingnothing” 71.

The master processor 31 then calculates 78 an average of “send_time” and“received_time,” and stores the calculated value as“initiators_average_time” 81, which is the time that a perfectlysynchronized clock decoder would have returned as its “responders_time.”Additionally, the master processor 31 accumulates 82, for each endpointcomponent processor 40, a running total or “overall_sum” 83 of the timeerrors between “responders_time” and “initiators_average_time.”

After iterating “n” times through determining 69 the time errors 68, themaster processor 31 averages 86 the “overall_sum” for each endpointcomponent processor 40 to generate a “correction_amount” 87 for eachclock decoder. Now that the “correction_amount” has been determined foreach endpoint component processor 40, the master processor 31 then sends88 to each endpoint component processor 40 an “adjust” packet 89, whichencodes the “correction_amount” 87 for setting the component counter 43to match the master counter 37. Each endpoint component processor 40increments 90 its component counter 43 according to the“correction_amount,” thereby setting the component counter 43approximately equal to a “basis_time” at which the “adjust” packet 89was sent, plus the “correction_amount.”

Referring again to FIG. 4, each endpoint component processor 40 thenreturns 92 a “returnadjust” packet 93 indicating the time value of theupdated component counter 43. The master processor 31 checks 94 whetherthe “returnadjust” packet time substantially matches the master counter37 minus the “correction_amount.” (In certain embodiments, the step ofchecking 94 may allow for various tolerances around the exact match;e.g., plus or minus several clock pulses, one microsecond, onemillisecond, or one one hundredth of a second). If so, then the masterprocessor 31 sends 96 a “syncresponder” packet 97, which confirms to theendpoint component processor 40 that its component counter 43 iscorrectly set. If not, then the master processor 31 sends 88 another“adjust” packet 89. Once all the clock decoders 42 have acknowledged 98their respective “syncresponder” packets by sending“returnsyncresponder” packets 99, the master processor 31 sends 100 apacket 101 “unsquelchallnodes,” which releases the receiving componentsto continue their normal operations. The master processor 31 alsoreports 102 back to the gantry control board 24 that sync wassuccessful.

Although the sync process 50 has been described as implemented in themaster processor 31 and in the endpoint component processors 40,variants or portions of the sync process 50 may be implemented in otherhardware, e.g., in the master switch 32 or specifically in the clockencoder 36.

Thus, aspects of the invention provide a method for synchronizing aplurality of components that are networked via a plurality of high speedswitches. The method comprises frequency-locking to a master clock thecomponent clocks of the plurality of components, and synchronizing to amaster counter, driven by the master clock, a plurality of componentcounters driven by the plurality of component clocks, so that thefrequency-locked component clocks drive the component counters insynchrony with the master counter. For example, frequency-locking maycomprise encoding a sequence of master clock pulses from the masterclock onto a clock stream transmitted among the plurality of components,and at each of the component clocks decoding a sequence of componentclock pulses from the clock stream. Encoding a sequence of master clockpulses may comprise receiving an master stream at a clock encoder,receiving the sequence of master clock pulses at the clock encoder, andproducing the clock stream from the clock encoder by modulating themaster stream according to the sequence of master clock pulses. Decodinga sequence of component clock pulses may comprise at each componentclock receiving the clock stream and demodulating the clock stream toproduce the sequence of component clock pulses and to reproduce themaster stream. Synchronizing may comprise encoding in the clock streaman instruction to set each component counter to match the mastercounter; encoding in the clock stream a plurality of instructions foreach component to return a plurality of receipt times corresponding tothe plurality of instructions; determining for each of the componentcounters a correction amount based on the plurality of receipt timesreturned by the corresponding component; and encoding in the clockstream an instruction to set each component counter to match the mastercounter adjusted by the correction amount corresponding to thatcomponent counter. In certain aspects, after synchronizing the componentcounters to the master counter, the frequency-locked component clocksmaintain synchrony with zero overhead.

Other aspects of the invention provide a method for frequency-locking acomponent clock to a master clock. The method comprises defining a clockfrequency and a master counter by periodically providing a master clockpulse from the master clock to a master processor. A clock encoderdefines a clock stream by modulating the master clock pulse onto amaster stream generated by the master processor at a bit transition rategreater than the clock frequency, and transmits the clock stream fromthe clock encoder to a clock decoder portion of the component clock,which recovers clock pulses from the clock stream. The method mayfurther comprise maintaining at the master clock a master counter basedon the master clock pulse; inserting in the master stream a firstinstruction to reset a component counter; and inserting in the masterstream a second instruction to set the component counter to match themaster counter. The method may further comprise inserting in the masterstream a plurality of third instructions to obtain from the componentclock a plurality of receipt times of those third instructions; andcalculating, based on the plurality of receipt times of the plurality ofthird instructions, an average delay time from the master clock to thecomponent clock. The method may further comprise inserting in the masterstream a fourth instruction to set the component counter to match themaster counter incremented by the average delay time from the masterclock to the component clock. The master stream may, for example, encodeCT scan data.

Embodiments of the invention provide a synchronous network time system,which comprises a master clock configured to generate a sequence ofmaster clock pulses at a clock frequency; a master processor configuredto receive the master clock pulses, maintain a master counter of themaster clock pulses, and generate a master stream including instructionsfor initializing a component counter; a clock encoder configured toreceive the master stream and the master clock pulses, and generate aclock stream by modulating the master clock pulses onto the masterstream; a clock decoder configured to receive the clock stream, andextract from the clock stream the master stream and a sequence ofcomponent clock pulses; and a component clock configured to initializeand maintain a component counter based on the instructions from themaster processor and the component clock pulses. The synchronous networktime system may further comprise a first high speed data switchincorporating the clock encoder, and a second high speed data switchconnected in communication with the clock decoder. The first high speeddata switch may be configured to communicate the master stream to thesecond high speed data switch via the clock decoder, while the clockdecoder is configured to provide the component clock pulses to thesecond high speed data switch, and the second high speed data switch isconfigured to reproduce the clock stream from the master stream and thecomponent clock pulses. In certain embodiments, the master clock, themaster processor, and the first high speed data switch are disposed on agantry control board of a CT scanner, the master processor is configuredto generate the master stream as a master stream also including aplurality of instructions for coordinating a plurality of gantrycomponents of the CT scanner to perform at least one mode of CT scan,and the first high speed data switch is configured to transmit the clockstream to the plurality of gantry components; and the second high speeddata switch is installed in communication between the clock decoder anda second clock decoder, which is installed as part of the componentclock in communication with one of the plurality of gantry components.In certain embodiments, the clock decoder extracts the sequence ofcomponent clock pulses by demodulating the clock stream. In someembodiments, the master clock is configured to generate in the masterstream a first instruction to reset the component counter and a secondinstruction to set the component counter to match the master counter.The master clock may also be configured to generate in the master streama plurality of third instructions to obtain from the component clock aplurality of receipt times corresponding to the plurality of thirdinstructions, and is further configured to calculate, based on theplurality of receipt times, an average delay time from the master clockto the component clock. Additionally, the master clock may be configuredto generate in the master stream a fourth instruction to set thecomponent counter to match the master counter incremented by the averagedelay time from the master clock to the component clock. The synchronousnetwork time system may further comprise a phase-locked loop configuredto remove jitter from the component clock pulses. In some embodiments,the component clock is configured to drive the component counter insynchrony with the master counter with zero overhead.

It is to be understood that the above description is intended to beillustrative, and not restrictive. For example, the above-describedembodiments (and/or aspects thereof) may be used in combination witheach other. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the inventionwithout departing from its scope. While the dimensions and types ofmaterials described herein are intended to define the parameters of theinvention, they are by no means limiting and are exemplary embodiments.Many other embodiments will be apparent to those of skill in the artupon reviewing the above description. The scope of the invention should,therefore, be determined with reference to the appended claims, alongwith the full scope of equivalents to which such claims are entitled. Inthe appended claims, the terms “including” and “in which” are used asthe plain-English equivalents of the respective terms “comprising” and“wherein.” Moreover, in the following claims, terms such as “first,”“second,” “third,” “upper,” “lower,” “bottom,” “top,” etc. are usedmerely as labels, and are not intended to impose numerical or positionalrequirements on their objects. Further, the limitations of the followingclaims are not written in means-plus-function format and are notintended to be interpreted based on 35 U.S.C. § 112, sixth paragraph,unless and until such claim limitations expressly use the phrase “meansfor” followed by a statement of function void of further structure.

This written description uses examples to disclose several embodimentsof the invention, including the best mode, and also to enable one ofordinary skill in the art to practice embodiments of the invention,including making and using any devices or systems and performing anyincorporated methods. The patentable scope of the invention is definedby the claims, and may include other examples that occur to one ofordinary skill in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral language of the claims.

As used herein, an element or step recited in the singular and proceededwith the word “a” or “an” should be understood as not excluding pluralof the elements or steps, unless such exclusion is explicitly stated.Furthermore, references to “one embodiment” of the present invention arenot intended to be interpreted as excluding the existence of additionalembodiments that also incorporate the recited features. Moreover, unlessexplicitly stated to the contrary, embodiments “comprising,”“including,” or “having” an element or a plurality of elements having aparticular property may include additional such elements not having thatproperty.

Since certain changes may be made in the above-described embodiments,without departing from the spirit and scope of the invention hereininvolved, it is intended that all of the subject matter of the abovedescription or shown in the accompanying drawings shall be interpretedmerely as examples illustrating the inventive concept herein and shallnot be construed as limiting the invention.

What is claimed is:
 1. A method for frequency-locking a component clockto a master clock, comprising: defining a clock frequency and a mastercounter by periodically providing a master clock pulse from the masterclock to a master processor; in a clock encoder, defining a clock streamby modulating the master clock pulse onto a master stream generated bythe master processor at a bit transition rate greater than the clockfrequency; and transmitting the clock stream from the clock encoder to aclock decoder portion of the component clock, which demodulates theclock stream to reproduce the master clock stream and obtains a sequenceof component clock pulses.
 2. The method as claimed in claim 1, furthercomprising: maintaining at the master clock a master counter based onthe master clock pulse; inserting a first instruction to reset acomponent counter; and inserting a second instruction to set thecomponent counter to match the master counter.
 3. The method as claimedin claim 2, further comprising: inserting a plurality of thirdinstructions to obtain from the component clock a plurality of receipttimes of those third instructions; and calculating, based on theplurality of receipt times of the plurality of third instructions, anaverage delay time from the master clock to the component clock.
 4. Themethod as claimed in claim 3, further comprising: inserting a fourthinstruction to set the component counter to match the master counterincremented by the average delay time from the master clock to thecomponent clock.
 5. The method as claimed in claim 1, wherein the masterstream encodes CT scan data.